The present invention relates to television signal switching systems of the type normally employed in television studio equipment to provide various types of television displays involving portions of more than one video input signal, or transitions from one video input signal to another.
Television signals may be broadly classified into two groups, namely, analog and digital. The majority of present day signal sources generate analog signals but there is an increasing use of equipment which generates digital signals.
Analog television (or video) signals are those in which a continuously varying voltage represents the picture information, both its brightness (or luminance) and its color content (or chrominance), the latter commonly being in the form of a modulated subcarrier added to the luminance signal.
Digital television signals are those in which the value of the signal representing picture information is sampled at specified times and this value is represented by a number, typically in binary code. These binary coded signals may then be handled by well-known circuit techniques. The value which is sampled may be that of the composite signals containing both luminance and chrominance information, or, alternatively, the luminance and chrominance components may be separated before they are sampled and converted to digital numbers.
Present day equipment for switching television signals are designed to handle analog signals. Consequently the outputs of digital sources have to be converted to analog form for switching and if digital output signals are required the switcher output must be reconverted from analog back to digital form.
Equipment and circuits for handling analog signals are prone to introduce various forms of distortion of the signal. In contrast, in digital equipment circuits can be designed to avoid these distortions. However, digital systems are limited to a discrete range of whole numbers, that is, they represent variations in the input as a series of steps and cannot distinguish input values lying between these steps. Such values will be represented by the nearest whole number value. Thus, a signal converted from analog form to digital form and then back to analog form will not be precisely identical to the original analog signal, but will contain errors where intermediate input values have been rounded off to the nearest digital whole number value. Errors are also introduced when a digital input is converted to analog form and then back to digital form because of small variations in gain and level and also of nonlinear distortions introduced by the analog circuits. These errors can result in the output digital number differing from the input digital number.
Repeated conversions of a signal between digital and analog forms are thus undesirable because of the degradation introduced by these conversions. It is highly desirable that digital signals should be processed in digital form and only converted to analog when this is to be the final output form with no requirement for reconversion to digital signals.